Recirculating-ball style steering systems are used on many types of automobiles, typically on heavier vehicles such as trucks and sport utility vehicles. The recirculating-ball style steering system usually consists of a gearbox that houses a worm gear that is rotatably linked to a steering wheel on one end and is connected to a ball-nut rack on the other end. The worm gear is similar to a ball screw, the threads are filled with ball bearings that recirculate through the worm gear and ball-nut rack as the worm gear turns. The ball bearings are used to reduce friction and unwanted excess clearance between the worm gear and the ball-nut rack. The ball-nut rack includes a set of rack-teeth that are in mesh with a sector gear, having involute-teeth. A sector shaft may extend from the sector gear and be rotatably linked to a pitman arm, which may in turn be connected to other steering components to steer road wheels of the automobile. When the steering wheel is rotated, the worm gear also rotates which moves the ball-nut rack in a linear direction. The recirculating-ball style gearbox provides a mechanical advantage from the steering wheel to the steerable road wheels. The rack-teeth of the ball-nut rack in turn rotate the sector gear and sector shaft. The rotation of the sector shaft in turn rotates the pitman arm, the pitman arm moves a track rod and set of tie rods which rotate steerable road wheels of the automobile.
Variable ratio steering has been employed in recirculating-ball style steering systems. Variable ratio steering means that the ratio between the rotation of steering wheel and the rotation of the steerable road wheels is not held constant throughout the entire rotation of the steering wheel. Depending on the position of the steering wheel, the road wheels may be more or less sensitive to rotation of the steering wheel. Typically in a recirculating-ball style steering system having variable ratio steering, the steering ratio is higher closer to the maximum turning positions (left or right) than when the steering wheel is in a neutral position and the road wheels are straight. The purpose of the variable ratio steering is to create greater mechanical advantage for the driver and reduce steering sensitivity when the steering wheel is close to its maximum turning positions. This lowers hand wheel parking efforts and gives the operator greater control while parallel parking, which is essential in large trucks and sport utility vehicles.
Recirculating-ball style steering systems have also utilized sector gears having involute-teeth that are tapered, but not in combination with variable ratio racks. Tapered involute-teeth increase the precision, convenience, and cost-effectiveness of setting the initial gear mesh load and mesh load maintenance throughout the life of the gear. Because of the tapered feature of the teeth, the sector gear can be adjusted in the longitudinal direction of the sector shaft to set and adjust the mesh load, which is a fairly simple procedure. On the other hand, setting mesh loads for recirculating-ball style steering systems with straight non-tapered sector teeth is cumbersome, expensive, and lacks the ability for future adjustments.
Tapered sector gears, however, have not been compatible with variable ratio racks. The tapered involute-teeth of the sector gears have had interference issues with variable ratio rack-teeth at the lower end of the tooth taper and gap issues at the upper end of the tooth taper.
It would be desirable to provide a recirculating-ball style steering system that includes both a variable ratio ball-nut rack and a tapered sector gear, to allow for variable ratio steering, precise and cost effective setting of initial gear mesh load, and the ability for future adjustment of the mesh load during the life of the vehicle.